Laminated glazings can be used as parts of alarm systems and, for this purpose, have fine electrical wires embedded in an interlayer of thermoplastic material which are connected to an electrical alarm device. If the glazing is broken, the resistance of the wires is altered, thereby activating the alarm device.
Such laminated glazings can also be used in another application, that of heated glazings. In this case, the wires embedded in the interlayer are connected to an electrical power source and the current passing through the wires raises the temperature of the glazing. Fogging of the glazing is avoided or eliminated and de-icing can be readily accomplished. Glazings with such built-in heating elements can be used, for instance, in air, sea and ground vehicles such as trains and automobiles. They are usually used in windshields or rear windows, primarily of automobiles.
The prior glazings usually have at least three transparent or translucent layers, i.e., an interlayer made of thermoplastic material and at least two rigid layers, one on each side of the interlayer. When the glazing consists of three layers, the wire network is embedded in the interlayer, which is usually made of polyvinylbutyral. This network comprises one or more fine metal wires that are parallel to each other. The wires are either rectilinear or, more often, sinusoidal. The latter arrangement has various advantages: it eliminates the phenomena of optical diffraction; it increases the length of the wire for a given distance; and it permits better heat dissipation.
The rigid layers covering the polyvinylbutyral can by sheets of glass or sheets of a plastic material such as polymethylmethacrylate. By covering the polyvinylbutyral, these layers protect it from external conditions, such as atmospheric conditions to which it is highly sensitive.
The rigid exterior layers have a thickness greater than one millimeter and often are two or three millimeters in thickness because of production and strength requirements and also because of the need for good optical qualities. Consequently, the above-mentioned laminted glazings with built-in heating or alarm elements weight a considerable amount. This weight constitutes a serious disadvantage, especially in the use of these glazings for automobiles. It is a well known fact that one of the major concerns of car manufacturers is reducing car weight in order to make them more economical and, by reducing the weight in the top portion of the car, to lower the center of gravity in order to make the car safer.
Another disadvantage related to the substantial thickness of the layers forming the known laminated glazings with built-in heating elements in the undue amount of time needed by the electrical heating resistance to heat the glazing surface sufficiently to de-ice it or defog it. This can also be called "heating inertia" and is due to the mass and poor thermal conductivity of the glass or plastic material used.
Furthermore, the considerable thickness of the layers requires that the resistance wires be raised to relatively high temperatures in order to produce adequate results.